The BeforeDropDown event will fire before the dropdown is made visible. This event can be used to cancel the dropdown action.
It can be the name of an implemented function or explicit javascript statement(s). If the name of the function is used, then that event handler may contain 3 parameters.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2